1. Field of the Invention
The present invention relates to the elimination of various forms of frequency perturbations of a phase locked oscillator. More particularly, the present invention relates to the use of a synchronous demodulator to determine the correlation between undesired frequency fluctuations of a phase locked oscillator and the perturbation signal itself, and the consequent generation of a control signal to cancel such undesired perturbations in a closed loop manner.
2. Art Background
This invention relates to the problem of stabilizing the frequency of a phase locked oscillator which is in the presence of various influences which tend to alter the operating frequency of the oscillator. There are many situations where this condition occurs, but a common example arises with Transmit/Receive Keying in a communication system. FIG. 1 is a block diagram depicting such a system. A synthesized frequency is derived from the phase locked oscillator comprising a phase detector 1, loop amplifier 2, loop divider 3, and voltage controlled oscillator 4, and is used as the excitation for the transmitter 5. Such signal may, in some applications, be used as part of the receiver circuitry 6. Many communication systems alternate between Transmit and Receive modes (called "T/R Keying") at a periodic and rapid rate.
If properly designed, having phase locked the oscillator in such a system provides assurance that the center frequency of operation does not vary. However, a problem arises in the presence of perturbing influences of a high frequency nature. Specifically, the phase locked loop which stabilizes the oscillator can not eliminate frequency fluctuations arising from components of the perturbing influences which are well above the bandwidth of the phase locked loop. FIG. 2 is a block diagram depicting this condition. The resulting fluctuations are unabated by the phase locked loop and appear as variations or errors in the transmitted frequency. In systems employing the same oscillator in the receiver circuitry, variations and errors occur in the nominal receive frequency.
The perturbing influences in the application described above can be many, but generally include "Power Supply Pushing", "Load Pulling", and "Injection Pulling." The action of applying or removing the excitation from the transmit path 7 can alter the current demands in the power supply, slightly altering its voltage, and the intrinsic frequency sensitivity of the oscillator to the power supply voltage gives rise to "Power Supply Pushing." This can generally be mitigated by improved supply bypassing and filtering. The other two phenomena are more onerous. In general, some form of switch is used to remove or apply the excitation signal from the transmit path, and that action may present a slight impedance variation to the output of the oscillator. Depending upon the magnitude and phase angle of the impedance variation, the frequency of the oscillator may shift considerably ("Load Pulling"). Additionally, if there are signal paths whereby the intermittent signal from the transmit antenna can reach the oscillator itself , again depending on the magnitude and phase angle of the signal reaching the oscillator, the frequency of operation may shift considerably ("Signal Injection Pulling"). FIG. 3 depicts typical frequency pulling phenomena of this nature. Mitigation of these two phenomena generally requires extensive shielding, buffering, harmonic multipliers, etc., and it is often difficult and expensive to achieve sufficient containment. Accordingly, in situations such as the example just described, where T/R Keying (or other perturbation) occurs at a rate substantially above the bandwidth of the phase locked loop, the oscillator frequency fluctuations are very problematic.
As will be described below in more detail, a novel and simple solution to the above problems is provided by a key alteration to the above described standard circuitry.